Ring gear for an epicyclic reduction gear

ABSTRACT

A method for aligning toothing in an assembly of two half-ring gears is provided:
         an angular positioning pin is provided which is to be received in respective holes of the half-ring gears;   the holes are drilled on the first half-ring gear such that the first hole has a first cross-section that is smaller than the final cross-section thereof;   the angles between the teeth are compared between the two half-ring gears, and an angular difference between said angles of the half-ring gears is deduced therefrom;   the first hole is redrilled to the final cross-section, while the centre of the hole is angularly shifted by the value of the angular difference;   the pin is engaged in the holes, and the half-ring gears are then assembled using an interference fit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage of International Application No.PCT/FR2016/052651, filed on of Oct. 13, 2016, which claims the benefitof French Patent Application No. 1559918, filed Oct. 16, 2015, thecontents of each of which are incorporated herein by reference.

TECHNICAL FIELD

This invention relates to obtaining a ring gear to be mounted in anepicyclic reduction gear.

BACKGROUND

The epicyclic reduction gears are frequently used in reduction units ofaircraft turboprop engines, to allow a reduction in speed between thedrive line and the propeller.

The teeth are major elements of the reduction gear since they transferthe power from the input shaft to the output shaft.

Typically, an epicyclic reduction gear comprises a combination ofcoaxial elements, one or more of which are ring gears, planet carrierswhich rotate around the common axis and which carry one or more planetsmeshing with one or more sun gears.

And in some cases, it is useful that the ring gear or gears be made intwo parts which are assembled.

Thus, in an epicyclic or planetary gear train, on a herringbonetoothing, namely a toothing composed of two helical gears set inopposition so as to cancel the axial force as it is necessary that oneof these elements, the sun gears, the planet gears, or the ring gears,be in two parts.

For manufacturing reasons, it is preferable that the ring gear be in twoparts, that is to say in two half-ring gears, with the understandingthat the expression “two half-ring gears” means two annular rings, apriori identical, or essentially identical, but each has, coupled withthe common axis of the half-ring gears, a lesser thickness (for exampleby half) with respect to that of the final ring gear obtained by thecoaxial assembly of the two half-ring gears.

Given that the aim here is to ensure a zero radial clearance between thetwo half-ring gears in question, which imposes a interference fitbetween them, and that it is possible for a tooth of one of thehalf-ring gears not to be aligned with the corresponding tooth on thesecond half-ring gear, a problem therefore arises regarding the offsetof the teeth of the half-ring gears in relation to each other: how toassemble these half-ring gears assembled while ensuring the alignment oftheir teeth?

SUMMARY

It is with a view to providing a solution to this problem, within thecontext of a serial production, at reduced cost (in terms of responsetime) that according to the invention, in order to align the teeth ofsaid toothing, the following steps should be followed:

-   -   an angular positioning pin is provided for the half-ring gears        in relation to each other, the pin to be received in the first        and second respective holes of the said first and second        half-ring gears, must each have a final cross-section adapted to        the pin;    -   with reference to the pre-established manufacturing drawing, the        first and second holes are drilled such that the first hole has        a first cross-section that is smaller than the said final        cross-section on the first half-ring gear and that the second        hole has a second section equal to the said final cross-section        on the second half-ring gear,    -   for each half-ring gear:        -   Several equidistant teeth from each other are indexed, with            a first tooth which is closer to the hole of its respective            half-ring gear than the other teeth, to define a common            frame to both half-ring gears;        -   on each tooth indexed, the multidimensional coordinates,            with respect to the centre of the half-ring gear, of (at            least) a reference point are determined;        -   the determined multidimensional coordinates are compared            with theoretical coordinates of these same points from the            pre-established manufacturing drawing;        -   a difference is noted between the determined coordinates and            the theoretical coordinates for defining a positional            deviation of said points;        -   the first hole is re-drilled to said final cross-section,            positioning or centering it based on the positional            deviations of the reference points of each half-ring gear            such that, during assembly, each tooth indexed to the first            half-ring gear facing a tooth indexed from the second            (opposite) half-ring gear, parallel to said axis, according            to the common frame,        -   and the first and second half-ring gears are assembled by            tightly engaging said angular positioning pin in the first            and second holes, and by an interference fit of the            half-ring gears together.

This interference fit and the consideration of an offset due to toothingwith non-identical angular positions between the two half-ring gearswill secure the realization of a complete ring gear without axialmisalignment of its two half-toothing.

And only drilling the hole smaller in relation to the finish dimensionsin only one of the two half-ring gears, rather than carrying out thisoperation on both half-ring gears, has the advantage of enabling theangular resetting without multiplying any measures leading to a loss ofprecision in such resetting.

To further secure the attainment of a major zero radial clearance foroptimized performance of an epicyclic reduction gear, it is furtherrecommended that the assembly step of the first and second half-ringgears comprises a shrink fit assembly. This makes it even more useful totake into account a possible angular offset between the teeth of thehalf-ring gears, thereby justifying the above method. Indeed, onceshrink fitted:

-   -   there is a risk of rotating material pull-out,    -   the tools and the method to follow are complex (see below).

Another problem taken into account was that concerning how to determine,with safety, reproducibility and as simply and reliably as possible, thepositional deviation of said points on the first and/or second half-ringgears.

For this purpose, it is advisable to locate each reference point on boththe active flank and on the pitch circle of the half-ring gearconcerned.

We define:

-   -   the active flank of a tooth as the lateral face of the tooth or        only a part of such face which comes into contact with the teeth        of the mating gear,    -   and the pitch circle as the straight section of the pitch        cylinder; its diameter is the pitch diameter.

It is equally recommended that the coordinates of each reference pointbe three-dimensional, said reference point being then on a middle(axial) extension plane of the tooth concerned.

This will solve the difficulty of defining the area where to measure theposition of the tooth in relation to the hole of the guidance pin.Indeed, this area should be sufficiently accessible for the measuringdevice, this measurement should be repetitive and the measured valueshould be truly reliable.

In addition, it is advisable that before determining, on each indexedtooth, the multidimensional coordinates of the reference point(s), atleast three equidistant teeth from the other should be defined one, andpreferably four equidistant teeth from each other should be defined andthus distributed at 90° in pairs.

Advantageously, the measurements will then be averaged on each half-ringgear.

In this way, the accuracy and quality of the final assembly will besecured through optimized measurement accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

The various aspects of the solutions presented herein will, ifnecessary, be better understood and other details, characteristics andadvantages thereof will become apparent upon reading the followingdescription as a non-exhaustive example with reference to the appendeddrawings wherein:

FIG. 1 is a diagram of the situation of the teeth in a reduction gearfor a turbine engine propeller, the planet gear carrier which is fixedin relation to the frame, is not shown in the drawing,

FIG. 2 outlines the inside of a planetary or epicyclic reductiongearbox,

FIG. 3 is the initial step of a coaxial assembly (axis 190 of the ringgear) between a first and a second half-ring gears,

FIG. 4 outlines an assembly pin to angularly wedge together thehalf-ring gears,

FIGS. 5, 6, 7 partially show both half-ring gears

FIG. 8 shows a complete ring gear, assembled, shrink fitted,

and FIG. 9 shows the characteristic teeth marked for taking thepotential manufacturing difference between the two half-ring gears.

DETAILED DESCRIPTION

Placed between the turbine and the propeller axis, the role of thereduction gear is to reduce the speed of rotation of the propeller. FIG.1 thus shows very diagrammatically a planetary or epicyclic reductiongear 1. The planet carrier which is fixed with respect to the frame isnot shown in the drawing below. It shows at 3 the turbine (input) shaft,and at 5 and 7, respectively planet gears and a ring gear of theepicyclic reduction gear 1. The (output) shaft of the propeller is at 9.

FIG. 2 shows the turbine shaft 3, the central sun gear 11, the zone 5 ofthe planet gears, or planetary gears, and the peripheral ring gear 7with inner teeth, here helical.

In the example, it is an epicyclic gear train with herringbone toothing.

For manufacturing reasons, it was considered preferable for the ring 7to be in two parts, that is to say in two half-ring gears 7 a, 7 b, asshown diagrammatically in FIG. 3.

The detail drawing of FIG. 5 outlines the embodiment of each of thesehalf-ring gears 7 a, 7 b, equally. Thus, each of them comprises an innerteeth, here helical, 13 over its entire inner periphery, just as what isillustrated with the same frame FIG. 1, such that it meshes with theouter teeth 15 of certain planet gears, at least.

A precise relative positioning of both half-ring gears 7 a, 7 b must beensured.

As shown diagrammatically in FIG. 5, a guidance pin 17, or an angularpositioning pin of the half-ring gears with respect to each other isprovided for this purpose. It will enable an alignment of the teeth ofboth half-ring gears.

To complete the effect provided by this pin, it is recommended that,once well positioned in relation to each other, with their teeth 13aligned, along with the (common, as in the example) axis 19 of theturbine shaft and the propeller 9, these first and second half-rings 7a, 7 b be assembled by a interference fit, and in this case by shrinkfit.

It should be recalled that a shrink fit consists of surrounding an innerpiece called the “enveloped part” by an outer part called the“enveloping part”. The assembly is made with machining tolerances thatprohibit its assembly by hand or even by press fit. One solution, whenpossible without deteriorating the material, is to heat the envelopingpart to expand it before putting it on the part to be enveloped.

Such a shrink fit assembly implies that, for the relative angularpositioning of the half-ring gears, it is no longer possible to rotatethem in relation to each other about their central axis, once shrinkfitted. Indeed, there is a risk of rotating material pull-out, and thetooling to be achieved is complex because the force required to ensurerelative rotation between the half-ring gears is therefore considerable,because of the shrink fitting. Moreover, this method would requireprecise control of the movements for the corrective rotations are weakafter the first positioning.

The solution proposed, in summary, is to drill, for the pin 17, in oneof the two half-ring gears a hole which is smaller than the dimension inthe plan, to measure the angular offset of the respective teeth of bothhalf-ring gears 7 a, 7 b, and finally to redrill the hole to thefinished dimensions, before the final assembly.

In more detail, the proposed method comprises the following steps:

Firstly, we will consider the angular positioning pin 17 of thehalf-ring gears 7 a,7 b in relation to each other, in that it will haveto be received in the first and second respective holes 21 a,21 b of thesaid first and second half-ring gears as these holes must each have afinal cross-section adapted to the pin.

The holes 21 a, 21 b will each be arranged, along an axis parallel tothe common axis 190 of the first and second half-ring gears.

Advantageously, they will each be arranged in a peripheral radial part23 of the half-ring gear concerned surrounding the inner toothing 13,substantially perpendicular to it.

A peculiarity lies in the fact that the first and second holes 21 a, 21b are drilled such that:

-   -   the first hole 21 a will have a first cross-section, such as S1        FIG. 6, smaller than said final cross-section on the first        half-ring gear 7 a,    -   and that the second hole 21 b has a second cross-section equal        to said final cross-section on the second half-ring gear 7 b.

Typically, the aforementioned sections may be diameters (holes with acircular cross-section).

Both holes 21 a, 21 b have been drilled at the same theoreticallocation.

As illustrated in FIG. 6 for a given tooth, each tooth, such as thatindexed 13 a, will have an active flank 25 and all the teeth of thefirst, respectively the second, half-ring gear will extend radially upto their area of the internal end section 130, with the same theoreticalpitch diameter indexed D1.

Having done that, we are going to define and mark, on each toothing,several teeth equidistant from each other, such as 13 a 1,13 a 2,13 a3,13 a 4 FIG. 9, with a first tooth which is closer to the hole (21 aFIG. 9) of its half-ring gear than the other teeth, so as to define acommon frame to both half-ring gears. Indeed, at the time of the finalassembly of both half-ring gears, it is important that they bepositioned co-axially, face to face, with the same relative angularposition of said “indexed teeth”.

Then, on each indexed tooth will be determined the multidimensionalcoordinates, with respect to the centre of the half-ring gear (axis190), of a reference point.

Three-dimensional coordinates (orthonormal coordinate system x, y, z)are a priori preferred to two-dimensional coordinates (x, y inparticular);

These multidimensional coordinates taken are then going to be comparedwith the theoretical coordinates of these same points from thepre-established manufacturing drawing.

In practice, this calculation of the theoretical coordinates would havebeen acquired much earlier by 3D survey from the pre-establishedmanufacturing drawing.

To present the foregoing otherwise, we can consider that, to measure thepossible offset between the herringbone toothing of both half-ringhears:

-   -   on the first half-ring gear, the one 7 a with the hole 21 a of        the guidance pin drilled smaller than the finished dimension:        -   a/ the aforementioned teeth 13 a 1, 13 a 2, 13 a 3, 13 a 4            will be indexed. The first tooth is therefore the tooth            closest to the hole 21 a of the guidance pin;        -   b/ with a three-dimensional spotting machine 28, a point            (marks 27 a 1, 27 a 3, FIGS. 6 and 9, respectively for the            teeth 13 a 1, 13 a 3) will be indexed or identified on each            of these teeth which will thus be located favourably on the            active flank, on the pitch circle (D1) and in the middle of            the tooth, that is to say at mid-axial length d/2, as shown            in the illustrations (see the middle (axial) extension plane            34 of the tooth in question FIG. 9).        -   c/ considering that the coordinates of said tooth will be            noted x1, y1, z1 in the associated computer 30, connected to            the data recording machine 28, thanks to 3D readings stored            in the machine memory 32 and initially made with reference            to the pre-established manufacturing drawing of said            half-ring gears, the theoretical position of each            aforementioned point marked (such as 27 a 1) will also be            available, its theoretical three-dimensional coordinates            being denoted x2, y2, z2;        -   d/ by calculating, using the calculator 30, the difference            between the coordinates x1, y1, z1 and x2, y2, z2, the gap E            of the coordinates (in the three axes x, y, z, in the            example explained here) between the theoretical position and            the actual position can therefore be determined.    -   on the second half-ring gear with the hole 21 b to the finished        dimensions, steps a) to d) above will be reproduced.

In this way, the said gap E on each half-ring gear in relation to thetheoretical position will be known, for the teeth (or half-teeth)concerned; that is to say the position envisaged by the manufacturingdrawing.

And the hole 21 a of the first half-ring gear can then be redrilled tothe finished dimensions by shifting so as to have between both half-ringgears the desired gap E.

Even if the aforementioned location of the holes 21 a, 21 b is “thesame” on both half-ring gears 7 a, 7 b if superimposed, it will thus bepossible to take into account the non-identity of the shapings of theirrespective teeth, which is usual in practice.

Once this is done, the first and second half-ring gears are assembledalong the common axis 190 by tightly engaging said pin 17 in the firstand second holes 21 a, 21 b, and by a peripheral interference fit of thehalf-ring gears between them, favourably by shrink fit.

To be sure of the quality of the inner teeth 13 of the half-ring gears 7a, 7 b, it is furthermore recommended that before any drilling is done,a rectification of these toothings should be carried out, on eachhalf-ring gear, separately. Thus, the teeth will have a precise andperfect appearance.

It should be understood that the essence is the search for an accuracyor precision in positioning the holes to be drilled and hence in that ofthe half-ring gears in relation to each other, angularly speaking, thatit is advisable to first calculate the gap E:

-   -   to perform measurements from three and preferably (at least)        four teeth separated in pairs by 90°, on each half-ring gear;    -   and then averaging these measurements, on each half-ring gear 7        a, 7 b, by calculating an arithmetic average.

Thus, in practice, it will be preferable, on each half-ring gear, tomeasure the gap between the theoretical position of four teeth at 90°and the actual position of these same teeth (see FIG. 9), then toarithmetically average the four differences obtained, the (“corrected”,if necessary) re-drilling of the hole of the guidance pin on thehalf-ring gear where it had been drilled smaller thereby cancelling anyangular error.

Once the holes 21 a, 21 b are correctly drilled, it will be possible totightly engage the pin 17 partially in each of these holes, thenassemble the first and second half-ring gears by the axially tight fitprovided.

FIG. 8 is a diagrammatic representation at 31 of the enveloping partsurrounding and axially tightening together the two peripheral radialparts 23 thus co-axially joined (axis 190) to form the final single ringgear 7.

The invention claimed is:
 1. A method for assembling first and secondhalf-ring gears according to a common axis, the method comprising:providing an angular positioning pin for the first and second half-ringgears in relation to each other, the angular positioning pin to bereceived in first and second respective holes of the said first andsecond half-ring gears, wherein the first and second respective holeseach have a final cross-section adapted to the angular positioning pin;with reference to a pre-established manufacturing drawing, drilling thefirst and second holes such that the first hole has a firstcross-section that is smaller than the final cross-section on the firsthalf-ring gear and that the second hole has a second cross-section equalto the said final cross-section on the second half-ring gear, for eachof the first and the second half-ring gears: indexing or defining aplurality of teeth equidistant from each other, with a first tooth thatis closer to the respective hole of its respective half-ring gear thanthe other teeth, to define a common index mark to the first and secondhalf-ring gears; determining on each tooth indexed, multidimensionalcoordinates with respect to a center of the respective half-ring gear,of a reference point; comparing the determined multidimensionalcoordinates with theoretical coordinates of these same points from thepre-established manufacturing drawing; noting a difference between thedetermined multidimensional coordinates and the theoretical coordinatesfor defining a positional deviation of said reference points; redrillingthe first hole to said final cross-section, positioning the first holebased on the positional deviations of the reference points of eachhalf-ring gear such that, during assembly, each tooth indexed to thefirst half-ring gear faces a tooth indexed from the second half-ringgear, parallel to said axis, according to a common frame, and assemblingthe first and second half-ring gears by tightly engaging said angularpositioning pin in the first and second holes, and by an interferencefit of the first and second half-ring gears together.
 2. The methodaccording to claim 1, wherein each reference point is located on both anactive flank and on a pitch circle of the respective half-ring gear. 3.The method according to claim 1, wherein the multidimensionalcoordinates of each reference point are three-dimensional, saidreference point being on a middle extension plane of the respectivetooth.
 4. The method according to claim 1, wherein the assembly step ofthe first and second half-rings comprises an interference fit assembly.5. The method according to claim 1, wherein before drilling the firstand second holes, the toothing on each half-ring gear is separatelyrectified.
 6. The method according to claim 1, wherein for eachhalf-ring gear, at least three teeth equidistant from each other aredefined.
 7. The method according to claim 1, wherein for each half-ringgear, four teeth equidistant from each other are defined and distributedat 90 degrees in pairs.